I get the same thing and it seems to be down to some form of interference from my Bluetooth transmitter, which again I get whether it is in use or not but just plugged in. The advice I got from a car audio 'expert', was that add on devices never work as well as hard-wired devices and That the car may require further 'modulation' or signal suppression as older cars are not as resistant to interference from local sources as newer cars. That's as far as I went with it, as I also thought he was trying to sell me a much more expensive solution, which he was and it would have probably worked, but I couldn't justify it in the car.